The invention relates to the field of plain old telephone services (POTS) and any type of digital subscriber loop (xDSL). More specifically the invention pertains to the transformers used in overlaid POTS and xDSL technology.
Digital subscriber loop (DSL) technology that offers the subscriber a very large bandwidth is engineered to overlay the existing analogue plain old telephone services (POTS). There are several types of DSL systems and the notation for specifying a DSL system of any type is xDSL. The xDSL system requires minimal equipment retrofit. It can be installed very quickly and easily, and is a cost-effective solution for high-bandwidth requirements. xDSL uses the existing copper analogue loop between the central office (CO) and the customer premises equipment (CPE) as its transmission medium, transporting voice in the traditional 4 kHz bandwidth where it has always been, while higher bandwidth digital services are relegated to higher frequency domains. A specific problem faced by overlaid POTS and xDSL technology, as well as other high-performance transmission systems, is the need to keep circuit costs low and packaging density high. Current technology makes use of a transformer for POTS and a second transformer for xDSL services in combination with a low pass filter (LPF) to combine POTS and xDSL services. The transformers have a significant volume and contribute significantly to the cost. Solutions for reducing the cost, volume and number of components in these circuits are therefore sought in the industry. Reducing the number of components also results in a reduction in inventory. In addition, due to the cost of real estate, an increased line density is required and would result in most of the savings per line (per customer) by sharing common equipment costs among a large number of lines.
The current technology involved with overlaying any type of digital subscriber loop (xDSL) services with plain old telephone services (POTS) makes use of two separate transformers, one for POTS and another for xDSL. In this invention the POTS transformer and the xDSL transformer are combined into one transformer. This is achieved by choosing a special geometric form for the core and by choosing strategic locations for the windings. A portion of the core is dedicated to serve as a shunt for each component of the magnetic field produced by the inductors. The geometric form of the core also provides a closed circuit of high permeability to restrict, to the core, the magnetic field produced by the current in the inductors. The windings of strongly coupled inductors are wound around the same portion of the core whereas the windings of weakly coupled inductors are wound around different core portions which are separated by the shunt. In overlaid POTS and xDSL applications a weak magnetic coupling between any one of the inductors used for POTS and any one of those used for xDSL services can therefore be achieved, despite their close proximity, by arranging the windings used for POTS and those of xDSL services around different portions of the core. Combining two transformers into one considerably reduces the weight volume and cost of the POTS+xDSL circuits.
In accordance with a first broad aspect, a transformer consists of a core formed by two coil portions, one central portion and connecting portions such that the central portion is spaced between the two coil portions and the connecting portions interconnect both ends of the central portion with the corresponding ends of the two coil portions. There is at least one first primary winding and at least one first secondary winding wound around one of the two coil portions of the core. There is also at least one second primary winding and at least one second secondary winding wound around the other coil portion of the core. The central portion is adapted to provide a shunt for components of the magnetic field produced by electric current in the windings.
The transformer may have two first primary windings and one first secondary winding. It may also have two second primary windings and one second secondary winding. The turn ratio of either of the two first primary windings to the first secondary winding may be 1:1 and the turn ratio of either of the two second primary windings to the second secondary winding may be 1:1. The transformer may be connected such that the two first primary windings are connected to a capacitor and connected to two conductors of a copper analogue loop, which is used to connect subscriber customer premise equipment (CPE) to a central office (CO). The two conductors are referred to as TIP and RING. The two first primary windings and the capacitor may form a high pass filter (HPF) and the first secondary winding may be connected to any type of digital subscriber loop (xDSL) circuit. The two first primary windings may be connected to one side of a low pass filter (LPF). The two second primary windings may be connected to another side of the LPF and the second secondary windings may be connected to a plain old telephone service (POTS) circuit.
The core of the transformer may be iron, laminated iron, powdered iron, ferrite or any other suitable magnetic material.
The cross-sectional area of the central portion of the transformer, in relation to the cross-sectional of its coils and connecting portions, may be specified to regulate the extent to which components of the magnetic field produced by electric currents in the windings are shunted through the central portion. In this way the strength of the magnetic coupling between any one of the first windings and any one of the second windings can be tuned by controlling the extent to which the components of the magnetic field are shunted through the central portion.
Each portion of the transformer may be a rectangular parallelepiped. The approximate width, height and depth of the central portion may be 12 mm, 12 mm and 6 mm, respectively. The approximate width, height and depth of the coil portions may be 1.5 mm, 12 mm and 6 mm, respectively. Finally, the approximate width, height and depth of the connecting portions may be 3 mm, 1.5 mm and 6 mm, respectively. The magnetic coupling between any one of the first windings and any one of the second windings may be in the range 0.01 to 0.25, whereas the magnetic coupling between any two first windings or any two second windings may be in the range 0.9 to 0.9999.
An air gap may be inserted into the core between the first primary and secondary windings of the transformer and its thickness may be approximately 0.1 mm. A second air gap may also into the core between the second primary and secondary windings of the transformer and its thickness may be approximately 0.1 mm.
The invention makes use of a combined POTS and xDSL transformer to reduce the cost and volume of the overlaid POTS and xDSL circuits. The combined transformer results in a reduced inventory. In addition, the combined transformer allows an increased line density which results in savings per line (per customer) by sharing common equipment costs among a large number of lines.